fix code completion when enableTrailingCommas is true

pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart

Index: pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart
index bbf9cb5ba7050b300e97c35691b283c15b63b15d..7e7ae5cb97189d10d70c5ef170eea38fe99f44e2 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_target.dart
@@ -22,6 +22,13 @@ int _computeArgIndex(AstNode containingNode, Object entity) {
     if (args.isEmpty) {
       return 0;
     }
+    if (entity == argList.rightParenthesis) {
+      // Parser ignores trailing commas
+      if (argList.rightParenthesis.previous?.lexeme == ',') {
+        return args.length;
+      }
+      return args.length - 1;
+    }
   }
   return null;
 }